This invention relates to an automatic hydraulic reversing valve for a double-action working cyliner having a control piston operating against a return spring, with the capability of applying pressure to the other side of the piston hydraulically from a relief pressure chamber, the relief pressure chamber communicating on one side with a working connection of the working cylinder through an intermediate relief valve and on the other side to a tank through a throttle orifice.
Double-action working cylinders which reverse automatially by means of a control valve are used in many industrial systems, such as trash balers, for example, which compress the trash by means of a pressure plate. In such applications, it is required for reasons of reliability for the working cylinder to operate completely hydraulically, i.e., without electrical limit switches which are more readily susceptible to contamination and thus problems. At the same time, for reasons of safety for the operating personnel, it should be guaranteed that the working cylinders always retract, i.e., open the system, when it is turned on, for example after an emergency stop.
Known valves for the control of such double-action working cylinders generally reverse in both directions automatically as a function of the pressure, but these are so-called servo-controlled valves, whose manufacturing costs are substantial because of the expensive servo-control mechanism incorporated therein. Valves of this type that do not incorporate servo-control mechanisms therein have been described in U.S. Pat. No. 2,711,717 in 1951; but because of deficient operating reliability, valves of this type have not come into use on the market.
The purpose of this invention is to provide a control valve for controlling a double-acting working cylinder which meets the specifications described above wherein it has a high operating reliability and is substantially more economical.
The objectives of the invention are obtained by the fact that during the response of the relief valve, the control valve can be pushed into a startup position by means of a separate thrust valve that is positioned between a pressurized chamber and an associated thrust valve end of the control valve, wherein the control valve is moved from a rest position in which a pressure connection is switched to an operating connection, with the control valve in the startup position opening because of negative overlapping of the operating connection to the thrust-piston side of the control piston. In this movement the thrust-piston side of the control piston is connected to the tank through a blocking slide valve mounted in the control piston, wherein the blocking slide valve is opened by the limit stop on the spring side of the control piston and is closed by the limit stop of the control piston on the thrust-piston side.
In the valve pursuant to the invention, the control valve is pressure-dependent, i.e., when the relief valve opens it is pushed from its rest position into a starting position in which a hydraulic medium, such as oil, reaches the thrust-piston side of the control piston because of the mentioned negative overlapping, and pushes the control piston smoothly and without interference up to its limit stop. When the control piston is moved in the opposite direction by its return spring, during which it must likewise pass through the mentioned negative overlap, then a blocking slide valve mounted in the control piston provides that the oil reaching the thrust-piston side during this return motion can flow out freely into the tank through the control piston, so that the negative overlap desired for the advance motion does not obstruct the return motion of the control piston.
In an especially preferred form of embodiment of the invention, the blocking slide valve is represented by an inner tube positioned coaxially in the control piston.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings: